1. Field of the Invention
The present invention relates to methods for manufacturing dielectric waveguides suitable for use in transmission lines and integrated circuits for use in the millimeter wave and microwave regions.
2. Description of the Related Art
Conventionally, a dielectric waveguide has a dielectric strip provided between a pair of conductor plates approximately parallel to each other for transmitting electromagnetic waves along the dielectric strip. In particular, a non radiative dielectric waveguide (hereinafter referred to as NRD guide) is a transmission waveguide having a small transmission loss in which a shielding area is formed by spacing a pair of conductor plates at a half or less of the wavelength of a transmitted wave so that no electromagnetic wave radiates from the dielectric strip. In the electromagnetic wave transmitting modes of the NRD guide, there are two types, i.e., an LSM mode and an LSE mode. The LSE mode, which has a smaller transmission loss, is generally used.
FIGS. 3 and 4 are cross-sectional views respectively showing two structures of conventional NRD guides. FIG. 3 shows the structure of a normal type NRD guide provided with a dielectric strip 53 between a pair of conductor plates 51, 52 disposed parallel to each other, which is disclosed in, for example, Japanese Examined Patent Application Publication No. 62-35281. FIG. 4 shows the structure of a so-called winged type NRD guide in which conductors 59, 60 are formed on external plane portions of dielectric strips 57, 58 having wing portions 55, 56, respectively, by a method, such as evaporation, or baking of silver paste, and in which the dielectric strip portions are disposed so as to oppose each other. The structure described above is disclosed in Japanese Unexamined Patent Application Publication No. 6-260814.
Compared to the normal type NRD guide, the winged type NRD has advantages in that the reproducibility of characteristics is superior, and the conductor and the dielectric strip thereof are easily aligned. In this connection, a synthetic resin, such as Teflon (registered trademark for PTFE, manufactured by E. I. du Pont de Nemours, Inc., U.S.A.), or a dielectric ceramic may be used, as the material for the dielectric strip. When a dielectric ceramic is used as a constituent material for the dielectric strip, since a dielectric ceramic generally has a higher relative dielectric constant than a synthetic resin, the bending loss can be decreased at a curved portion, and hence, miniaturization can be accomplished. Accordingly, development of dielectric strips using dielectric ceramics is currently progressed. The widths of the dielectric strips 57, 58, and the thicknesses of the wing portions 55, 56 are determined in accordance with the relative dielectric constant of a dielectric material to be used and the frequency of electromagnetic wave to be used. In general, when the relative dielectric constant is larger, and working frequency is higher, the widths w and the thicknesses t are decreased.
In a process for forming a winged type NRD guide as shown in FIG. 4 using a dielectric ceramic, a ceramic plate is preliminarily fired and polished, and then, as disclosed in Japanese Unexamined Patent Application Publication No. 10-224120, a plurality of green sheets having openings therein are laminated on the ceramic plate. Then, by firing the green sheet laminate, an NRD guide can be manufactured having a dielectric strip in a desired shape.
However, since a fired ceramic is very hard, a problem is that great time and effort are required for machining the fired ceramic plate so as to have a desired shape. In addition, since the thickness of the wing portion is small, another problem is that cracks and chips are likely to occur during machining.
In addition, in the method for laminating green sheets having openings therein, it is extremely difficult to accurately cut the green sheets in accordance with the width w of the dielectric strip and to accurately align the green sheets together. In the NRD guide used as a high frequency transmission waveguide in many cases, very high dimensional accuracy is required for the dielectric strip. Hence, there has been the problem in that the workability is poor.
Addressing these problems, the present invention provides a method for manufacturing a dielectric waveguide at lower manufacturing cost, in which the cracks and chips generated during machining in the conventional method are avoided, and in which a dielectric strip having accurate individual dimensions can be formed.
Through intensive research by the inventors of the present invention on the problems described above, it was discovered that the problems could be solved by a process comprising a step of forming a resist pattern on a green sheet containing a powdered inorganic material and an organic binder, a step of removing a predetermined amount of the green sheet corresponding to an opening of the resist pattern by the use thereof as a mask, a subsequent step of removing the resist pattern, and a step of firing the green sheet.
Thus, the present invention related to a method for manufacturing a dielectric waveguide including a pair of conductor plates approximately parallel to each other and a dielectric strip provided therebetween, in which the dielectric strip is formed by a process comprising a step of forming a resist pattern on a green sheet containing a powdered inorganic material and an organic binder, a step of removing a predetermined amount of the green sheet corresponding to an opening in the resist pattern by the use thereof as a mask, a step of removing the resist pattern, and a step of firing the green sheet.
According to the present invention, since it is not necessary to machine a fired hard ceramic plate as in the conventional example, and an unnecessary part of the green sheet is removed while it is in the green sheet state, cracks and chips are not generated, and thus, machining can be performed in a short period of time. In addition, since the dielectric strip is not formed by laminating a plurality of patterned thin green sheets, the conventional operation involving accurately aligning the green sheets is not required, and hence, the manufacturing process for the dielectric waveguide can be simplified. Furthermore, since a photolithographic technique which can perform accurate patterning can be applied to the patterning for the resist pattern, individual dimensions of the dielectric waveguide can be accurately defined, and hence, the dimensional accuracy can be significantly improved compared to the case in which the dimensions are defined by cutting.
For removing the green sheet, erosion processes can be used, such as sand blasting, slurry erosion, cavity erosion, sputtering, chemical milling, ion milling, and reactive ion etching (RIE). In this connection, xe2x80x9cerosionxe2x80x9d means a phenomenon in which the surface of a material is mechanically damaged by repetitive collisions (or impacts) of a fluid, and a part of the material is driven or plucked away (xe2x80x9cErosion and Corrosion,xe2x80x9d Japan Society of Corrosion Engineering Association, 1987, published by Shokabo Publishing Co., Ltd.). Among the processes mentioned above, sand blasting is most preferably used since a method using a vacuum process is not so suitable for performing fine machining of green sheets containing water and an organic component, and since high dimensional accuracy can be obtained by sand blasting in the formation of the dielectric strip which requires relatively deep etching, such as 0.2 to 1.0 mm.
In addition, when the green sheet is removed by a technique using erosion, the problem may arise in some cases in that side etching occurs when erosion progresses in the depth direction of the green sheet. That is, among the blasting particles contained in the fluid which collide with the surface of a material, some of the particles do not collide with the surface at right angles with respect to the surface of the material but rather have slanted incident angles and thereby are reflected toward the side of the surface of the green sheet. These particles etch the green sheet in the lateral direction thereof and thereby cause side etching. Furthermore, as the green sheet is removed in the depth direction, the part of the green sheet at which removal is performed at an initial stage is exposed to blasting particles for a longer period of time, whereby side etching is likely to occur particularly in the vicinity of the surface of the green sheet.
According to the present invention, side etching is constrained by using a green sheet in which the rate of removal by erosion is changed continuously or intermittently along the depth direction of the green sheet. That is, a part of the green sheet in the vicinity of the surface thereof, which is removed in an initial stage, is formed of a material having a high resistance to blasting compared to that inside the green sheet, in other words, the surface material has a low rate of removal by erosion, whereby the side etching is unlikely to occur even if the part of the green sheet described above is exposed to the blasting particles for a longer period of time.
With this continuously or intermittently changing rate of removal by erosion, the rate of removing the green sheet is gradually increased from the surface to the inside thereof along the depth direction. The change in rate of removal may be continuous or intermittent.
In order to change the rate of removal by erosion, there may be mentioned, for example, a method in which the content of a powdered inorganic material contained in the green sheet is changed along the depth direction thereof; a method in which the content of an organic binder contained in the green sheet is changed along the depth direction thereof; and the like.
In this connection, a step of removing the resist pattern and a step of firing the green sheet may be simultaneously performed. That is, when the green sheet is fired at a high temperature, the resist pattern may be removed by simultaneous pyrolysis thereof. As a result, the process can be simplified even more.
In addition, when the green sheet is removed by sand blasting or the like, in order to improve the workability and to prevent the deformation of the green sheet during the removing step, and the like, the removal is preferably performed after the green sheet is disposed on a fired hard ceramic base body. In the case described above, the ceramic base body can be the wing portion.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings, in which like references denote like elements and parts.